Web Authorization Protocol D. Fett
Internet-Draft yes.com
Intended status: Standards Track J. Bradley
Expires: January 9, 2020 Yubico
B. Campbell
Ping Identity
T. Lodderstedt
yes.com
M. Jones
Microsoft
July 8, 2019
OAuth 2.0 Demonstration of Proof-of-Possession at the Application Layer
draft-fett-oauth-dpop-02
Abstract
This document describes a mechanism for sender-constraining OAuth 2.0
tokens via a proof-of-possession mechanism on the application level.
This mechanism allows for the detection of replay attacks with access
and refresh tokens.
Status of This Memo
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 3
2. Main Objective . . . . . . . . . . . . . . . . . . . . . . . 3
3. Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. DPoP Proof JWTs . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Checking DPoP Proofs . . . . . . . . . . . . . . . . . . 6
5. Token Request (Binding Tokens to a Public Key) . . . . . . . 7
6. Resource Access (Proof of Possession for Access Tokens) . . . 8
7. Public Key Confirmation . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9.1. DPoP Proof Replay . . . . . . . . . . . . . . . . . . . . 9
9.2. Signed JWT Swapping . . . . . . . . . . . . . . . . . . . 10
9.3. Signature Algorithms . . . . . . . . . . . . . . . . . . 10
9.4. Message Integrity . . . . . . . . . . . . . . . . . . . . 10
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10.1. OAuth Access Token Type Registration . . . . . . . . . . 11
10.2. JSON Web Signature and Encryption Type Values
Registration . . . . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1. Normative References . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . 12
11.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Appendix A. Document History . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
[I-D.ietf-oauth-mtls] describes methods to bind (sender-constrain)
access tokens using mutual Transport Layer Security (TLS)
authentication with X.509 certificates.
[I-D.ietf-oauth-token-binding] provides mechanisms to sender-
constrain access tokens using HTTP token binding.
Due to a sub-par user experience of TLS client authentication in user
agents and a lack of support for HTTP token binding, neither
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mechanism can be used if an OAuth client is a Single Page Application
(SPA) running in a web browser.
This document outlines an application-level sender-constraining for
access tokens and refresh tokens that can be used if neither mTLS nor
OAuth Token Binding are available. It uses proof-of-possession based
on a public/private key pair and application-level signing.
DPoP can be used with public clients and, in case of confidential
clients, can be combined with any client authentication method.
1.1. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This specification uses the terms "access token", "refresh token",
"authorization server", "resource server", "authorization endpoint",
"authorization request", "authorization response", "token endpoint",
"grant type", "access token request", "access token response", and
"client" defined by The OAuth 2.0 Authorization Framework [RFC6749].
2. Main Objective
Under the attacker model defined in [I-D.ietf-oauth-security-topics],
the mechanism defined by this specification tries to ensure that
token replay at a different endpoint is prevented.
More precisely, if an adversary is able to get hold of an access
token or refresh token because it set up a counterfeit authorization
server or resource server, the adversary is not able to replay the
respective token at another authorization or resource server.
Secondary objectives are discussed in Section 9.
3. Concept
The main data structure introduced by this specification is a DPoP
proof JWT, described in detail below. A client uses a DPoP proof JWT
to prove the possession of a private key belonging to a certain
public key. Roughly speaking, a DPoP proof is a signature over some
data of the request to which it is attached to and a timestamp.
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+--------+ +---------------+
| |--(A)-- Token Request ------------------->| |
| Client | (DPoP Proof) | Authorization |
| | | Server |
| |<-(B)-- DPoP-bound Access Token ----------| |
| | (token_type=DPoP) +---------------+
| | PoP Refresh Token for public clients
| |
| | +---------------+
| |--(C)-- DPoP-bound Access Token --------->| |
| | (DPoP Proof) | Resource |
| | | Server |
| |<-(D)-- Protected Resource ---------------| |
| | +---------------+
+--------+
Figure 1: Basic DPoP Flow
The basic steps of an OAuth flow with DPoP are shown in Figure 1:
o (A) In the Token Request, the client sends an authorization code
to the authorization server in order to obtain an access token
(and potentially a refresh token). The client attaches a DPoP
proof to the request in an HTTP header.
o (B) The AS binds (sender-constrains) the access token to the
public key claimed by the client in the DPoP proof; that is, the
access token cannot be used without proving possession of the
respective private key. This is signaled to the client by using
the "token_type" value "DPoP".
o If a refresh token is issued to a public client, it is sender-
constrained in the same way. For confidential clients, refresh
tokens are bound to the "client_id", which is more flexible than
binding it to a particular public key.
o (C) If the client wants to use the access token, it has to prove
possession of the private key by, again, adding a header to the
request that contains a DPoP proof. The resource server needs to
receive information about which public key to check against. This
information is either encoded directly into the access token (for
JWT structured access tokens), or provided at the token
introspection endpoint of the authorization server (not shown).
o (D) The resource server refuses to serve the request if the
signature check fails or the data in the DPoP proof is wrong,
e.g., the request URI does not match the URI claim in the DPoP
proof JWT.
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o When a refresh token that is sender-constrained using DPoP is used
by the client, the client has to provide a DPoP proof just as in
the case of a resource access. The new access token will be bound
to the same public key.
The mechanism presented herein is not a client authentication method.
In fact, a primary use case is public clients (single page
applications) that do not use client authentication. Nonetheless,
DPoP is designed such that it is compatible with "private_key_jwt"
and all other client authentication methods.
DPoP does not directly ensure message integrity but relies on the TLS
layer for that purpose. See Section 9 for details.
4. DPoP Proof JWTs
DPoP uses so-called DPoP proof JWTs for binding public keys and
proving knowledge about private keys.
4.1. Syntax
A DPoP proof is a JWT ([RFC7519]) that is signed (using JWS,
[RFC7515]) using a private key chosen by the client (see below). The
header of a DPoP JWT contains at least the following parameters:
o "typ": type header, value "dpop+jwt" (REQUIRED).
o "alg": a digital signature algorithm identifier as per [RFC7518]
(REQUIRED). MUST NOT be "none" or an identifier for a symmetric
algorithm (MAC).
o "jwk": representing the public key chosen by the client, in JWK
format, as defined in [RFC7515] (REQUIRED)
The body of a DPoP proof contains at least the following claims:
o "jti": Unique identifier for this JWT chosen freshly when creating
the DPoP proof (REQUIRED). SHOULD be used by the AS for replay
detection and prevention. See Security Considerations [1].
o "http_method": The HTTP method for the request to which the JWT is
attached, as defined in [RFC7231] (REQUIRED).
o "http_uri": The HTTP URI used for the request, without query and
fragment parts (REQUIRED).
o "iat": Time at which the JWT was created (REQUIRED).
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An example DPoP proof is shown in Figure 2.
{
"typ": "dpop+jwt",
"alg": "ES256",
"jwk": {
"kty": "EC",
"crv": "P-256",
"x": "f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU",
"y": "x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0"
}
}.{
"jti": "HK2PmfnHKwXP",
"http_method": "POST",
"http_uri": "https://server.example.com/token",
"iat": 1555555555
}
Figure 2: Example JWT content for "DPoP" proof header.
Note: To keep DPoP simple to implement, only the HTTP method and URI
are signed in DPoP proofs. Nonetheless, DPoP proofs can be extended
to contain other information of the HTTP request (see also
Section 9.4).
4.2. Checking DPoP Proofs
To check if a string that was received as part of an HTTP Request is
a valid DPoP proof, the receiving server MUST ensure that
1. the string value is a well-formed JWT,
2. all required claims are contained in the JWT,
3. the "typ" field in the header has the value "dpop+jwt",
4. the algorithm in the header of the JWT indicates an asymmetric
digital signature algorithm, is not "none", is supported by the
application, and is deemed secure,
5. that the JWT is signed using the public key contained in the
"jwk" header of the JWT,
6. the "http_method" claim matches the respective value for the HTTP
request in which the JWT was received (case-insensitive),
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7. the "http_uri" claims matches the respective value for the HTTP
request in which the JWT was received, ignoring any query and
fragment parts,
8. the token was issued within an acceptable timeframe (see
Section 9.1), and
9. that, within a reasonable consideration of accuracy and resource
utilization, a JWT with the same "jti" value has not been
received previously (see Section 9.1).
Servers SHOULD employ Syntax-Based Normalization and Scheme-Based
Normalization in accordance with Section 6.2.2. and Section 6.2.3. of
[RFC3986] before comparing the "http_uri" claim.
5. Token Request (Binding Tokens to a Public Key)
To bind a token to a public key in the token request, the client MUST
provide a valid DPoP proof JWT in a "DPoP" header. The HTTPS request
shown in Figure 3 illustrates the protocol for this (with extra line
breaks for display purposes only).
POST /token HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded;charset=UTF-8
DPoP: eyJhbGciOiJSU0ExXzUi...
grant_type=authorization_code
&code=SplxlOBeZQQYbYS6WxSbIA
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
Figure 3: Token Request for a DPoP sender-constrained token.
The HTTP header "DPoP" MUST contain a valid DPoP proof.
The authorization server, after checking the validity of the token,
MUST associate the access token issued at the token endpoint with the
public key. It then sets "token_type" to "DPoP" in the token
response.
A client typically cannot know whether a certain AS supports DPoP.
It therefore SHOULD use the value of the "token_type" parameter
returned from the AS to determine support for DPoP: If the token type
returned is "Bearer" or another value, the AS does not support DPoP.
If it is "DPoP", DPoP is supported. Only then, the client needs to
send the "DPoP" header in subsequent requests and use the token type
"DPoP" in the "Authorization" header as described below.
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If a refresh token is issued to a public client at the token endpoint
and a valid DPoP proof is presented, the refresh token MUST be bound
to the public key contained in the header of the DPoP proof JWT.
If a DPoP-bound refresh token is to be used at the token endpoint by
a public client, the AS MUST ensure that the DPoP proof contains the
same public key as the one the refresh token is bound to. The access
token issued MUST be bound to the public key contained in the DPoP
proof.
6. Resource Access (Proof of Possession for Access Tokens)
To make use of an access token that is token-bound to a public key
using DPoP, a client MUST prove the possession of the corresponding
private key by providing a DPoP proof in the "DPoP" request header.
The DPoP-bound access token must be sent in the "Authorization"
header with the prefix "DPoP".
If a resource server detects that an access token that is to be used
for resource access is bound to a public key using DPoP (via the
methods described in Section 7) it MUST check that a header "DPoP"
was received in the HTTP request, and check the header's contents
according to the rules in Section 4.2.
The resource server MUST NOT grant access to the resource unless all
checks are successful.
GET /protectedresource HTTP/1.1
Host: resourceserver.example.com
Authorization: DPoP eyJhbGciOiJIUzI1...
DPoP: eyJhbGciOiJSU0ExXzUi...
Figure 4: Protected Resource Request with a DPoP sender-constrained
access token.
7. Public Key Confirmation
It MUST be ensured that resource servers can reliably identify
whether a token is bound using DPoP and learn the public key to which
the token is bound.
Access tokens that are represented as JSON Web Tokens (JWT) [RFC7519]
MUST contain information about the DPoP public key (in JWK format) in
the member "jkt#S256" of the "cnf" claim, as shown in Figure 5.
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The value in "jkt#S256" MUST be the base64url encoding [RFC7515] of
the JWK SHA-256 Thumbprint (according to [RFC7638]) of the public key
to which the access token is bound.
{
"iss": "https://server.example.com",
"sub": "something@example.com",
"exp": 1503726400,
"nbf": 1503722800,
"cnf":{
"jkt#S256": "oKIywvGUpTVTyxMQ3bwIIeQUudfr_CkLMjCE19ECD-U"
}
}
Figure 5: Example access token body with "cnf" claim.
When access token introspection is used, the same "cnf" claim as
above MUST be contained in the introspection response.
Resource servers MUST ensure that the fingerprint of the public key
in the DPoP proof JWT equals the value in the "jkt#S256" claim in the
access token or introspection response.
8. Acknowledgements
We would like to thank David Waite, Filip Skokan, Mike Engan, and
Justin Richer for their valuable input and feedback.
This document resulted from discussions at the 4th OAuth Security
Workshop in Stuttgart, Germany. We thank the organizers of this
workshop (Ralf Kuesters, Guido Schmitz).
9. Security Considerations
The Prevention of Token Replay at a Different Endpoint [2] is
achieved through the binding of the DPoP proof to a certain URI and
HTTP method. However, DPoP does not achieve the same level of
protection as, for example, OAuth Mutual TLS [I-D.ietf-oauth-mtls],
as described in the following.
9.1. DPoP Proof Replay
If an adversary is able to get hold of a DPoP proof JWT, the
adversary could replay that token later at the same endpoint (the
HTTP endpoint and method are enforced via the respective claims in
the JWTs). To prevent this, servers MUST only accept DPoP proofs for
a limited time window after their "iat" time, preferably only for a
brief period. Furthermore, the "jti" claim in each JWT MUST contain
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a unique (incrementing or randomly chosen) value, as proposed in
[RFC7253]. Resource servers SHOULD store values at least for the
time window in which the respective JWT is accepted and decline HTTP
requests by clients if a "jti" value has been seen before.
Note: To acommodate for clock offsets, the server MAY accept DPoP
proofs that carry an "iat" time in the near future (e.g., up to one
second in the future).
9.2. Signed JWT Swapping
Servers accepting signed DPoP proof JWTs MUST check the "typ" field
in the headers of the JWTs to ensure that adversaries cannot use JWTs
created for other purposes in the DPoP headers.
9.3. Signature Algorithms
Implementers MUST ensure that only digital signature algorithms that
are deemed secure can be used for signing DPoP proofs. In
particular, the algorithm "none" MUST NOT be allowed.
9.4. Message Integrity
DPoP does not ensure the integrity of the payload or headers of
requests. The signature of DPoP proofs only contains the HTTP URI
and method, but not, for example, the message body or other request
headers.
This is an intentional design decision to keep DPoP simple to use,
but as described, makes DPoP potentially susceptible to replay
attacks where an attacker is able to modify message contents and
headers. In many setups, the message integrity and confidentiality
provided by TLS is sufficient to provide a good level of protection.
Implementers that have stronger requirements on the integrity of
messages are encouraged to either use TLS-based mechanisms or signed
requests. TLS-based mechanisms are in particular OAuth Mutual TLS
[I-D.ietf-oauth-mtls] and OAuth Token Binding
[I-D.ietf-oauth-token-binding].
Note: While signatures on (parts of) requests are out of the scope of
this specification, signatures or information to be signed can be
added into DPoP proofs.
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10. IANA Considerations
10.1. OAuth Access Token Type Registration
This specification registers the following access token type in the
OAuth Access Token Types registry defined in [RFC6749].
o Type name: "DPoP"
o Additional Token Endpoint Response Parameters: (none)
o HTTP Authentication Scheme(s): Bearer
o Change controller: IETF
o Specification document(s): [[ this specification ]]
10.2. JSON Web Signature and Encryption Type Values Registration
This specification registers the "dpop+jwt" type value in the IANA
JSON Web Signature and Encryption Type Values registry [RFC7515]:
o "typ" Header Parameter Value: "dpop+jwt"
o Abbreviation for MIME Type: None
o Change Controller: IETF
o Specification Document(s): [[ this specification ]]
11. References
11.1. Normative References
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
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[RFC7253] Krovetz, T. and P. Rogaway, "The OCB Authenticated-
Encryption Algorithm", RFC 7253, DOI 10.17487/RFC7253, May
2014, <https://www.rfc-editor.org/info/rfc7253>.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015,
<https://www.rfc-editor.org/info/rfc7518>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC7638] Jones, M. and N. Sakimura, "JSON Web Key (JWK)
Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September
2015, <https://www.rfc-editor.org/info/rfc7638>.
11.2. Informative References
[I-D.ietf-oauth-mtls]
Campbell, B., Bradley, J., Sakimura, N., and T.
Lodderstedt, "OAuth 2.0 Mutual TLS Client Authentication
and Certificate-Bound Access Tokens", draft-ietf-oauth-
mtls-15 (work in progress), July 2019.
[I-D.ietf-oauth-security-topics]
Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
"OAuth 2.0 Security Best Current Practice", draft-ietf-
oauth-security-topics-13 (work in progress), July 2019.
[I-D.ietf-oauth-token-binding]
Jones, M., Campbell, B., Bradley, J., and W. Denniss,
"OAuth 2.0 Token Binding", draft-ietf-oauth-token-
binding-08 (work in progress), October 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <https://www.rfc-editor.org/info/rfc7515>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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11.3. URIs
[1] #Security
[2] #Objective_Replay_Different_Endpoint
Appendix A. Document History
[[ To be removed from the final specification ]]
-02
o added normalization rules for URIs
o removed distinction between proof and binding
o "jwk" header again used instead of "cnf" claim in DPoP proof
o renamed "Bearer-DPoP" token type to "DPoP"
o removed ability for key rotation
o added security considerations on request integrity
o explicit advice on extending DPoP proofs to sign other parts of
the HTTP messages
o only use the jkt#S256 in ATs
o iat instead of exp in DPoP proof JWTs
o updated guidance on token_type evaluation
-01
o fixed inconsistencies
o moved binding and proof messages to headers instead of parameters
o extracted and unified definition of DPoP JWTs
o improved description
-00
o first draft
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Authors' Addresses
Daniel Fett
yes.com
Email: mail@danielfett.de
John Bradley
Yubico
Email: ve7jtb@ve7jtb.com
Brian Campbell
Ping Identity
Email: bcampbell@pingidentity.com
Torsten Lodderstedt
yes.com
Email: torsten@lodderstedt.net
Michael Jones
Microsoft
Email: mbj@microsoft.com
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